Generally speaking, this invention relates to thermal printers characterized by having exothermic printing elements driven by pulses of current which heat the exothermic elements to effect printing. In the art, such conventional thermal printing units are provided with a central processing unit which performs calculations and other functions and a printer control unit which controls the application of pulses to the exothermal printing elements in response to the signals received from the central processing unit. Generally, in the art, timing for clocking the central processing unit and for controlling the frequency and period of the printing pulses is controlled by a single oscillator. Since such thermal printers are frequently used in conjunction with battery-driven devices, and since the voltage of battery voltage sources varies with time, it was necessary to provide voltage regulator circuitry for stabilizing the voltage applied to the exothermic elements to effect printing. Voltage stabilization is generally selected at a level below the minimum voltage to which the battery may fall, thereby not efficiently using the full power available to the device.
Further, if the voltage regulator is damaged, a higher voltage than the voltage for which the exothermic printing elements are designed is applied to such exothermic printing elements, consequently resulting in the burning and damaging thereof. For this reason, the conventional thermal printers suffered from inferior reliability.
A further difficulty arises from the fact that the prior art thermal printers were frequently manufactured by entities different from the manufacturers of the electronic table calculators to which they were to be joined and were adapted to operate at different predetermined applied voltages, making proper operation of the prior art thermal printers more difficult and more expensive. A further difficulty in thermal printers is unevenness of printing due to variations in operating temperature due to variations in ambient temperature.
The foregoing difficulties in the prior art are avoided by providing a thermal printer wherein the pulse width of the signal applied to the exothermic printing elements is controlled in response to input voltage and ambient temperature.